One of the major problems in development and aging is what we do not understand the nature of the "developmental program" through which a cell passes between birth and death. Insect metamorphosis involves a conspicious program with many advantages for analysis. We have developed culture conditions which permit lepidopteran pupal wings to form adult cuticle after ecdysone stimulation and pupal cuticle when juvenile hormone is also present. Comparable studies with imaginal disks revealed that some regions have the potentiality of by-passing the pupal stages and forming adult scales. The basis for this regional difference as under investigation. We have done amino acid analyses of a few of the cuticular proteins and found that some of the electrophoretic bands have similar amino acid compositions and may belong to the same protein family. We also have evidence for post-translational modifications. We are now engaged in determining why proteins made in vitro by ecdysone-stimulated epidermis do not have the same electrophoretic mobility as cuticular proteins made in vivo.